My invention comprises improvements in the inventions I have disclosed in U.S. Pat. No. 4,760,892 and in U.S. patent application Ser. No. 209,162, filed June 17, 1988, which are assigned to the assignee of this invention. These earlier disclosures relate to a power steering system having a positive displacement pump driven by a vehicle engine.
The pump used in the system of my invention, as in conventional power steering systems, is a positive displacement pump with a flow control valve that establishes a constant flow delivery regardless of the driven speed of the pump. A driven member, such as a rack in a steering gear linkage mechanism for the dirigible wheels of a vehicle, is actuated by a pinion that is connected to a driver controlled torque input shaft through a torsion bar. The input shaft is connected also to an inner valve member, and the pinion is connected to a valve sleeve surrounding the inner valve member. The relative displacement of the inner valve member relative to the sleeve is proportional to the deflection of the torsion bar.
The rotary valve assembly is situated in and partly defines a pressure distributor circuit between the pump and opposed working chambers of a fluid motor that actuates the steering gear rack.
In my earlier disclosures I have described a means for distributing pump flow made available to the steering gear rotary control valve assembly using either a pressure regulator valve or an area control valve that is under the influence of an electronic actuator. A vehicle speed sensor and a steering torque sensor provide control signals to a central processor unit in the form of an electronic processor. The output signal from the processor is amplified and used to energize the electronic actuator, usually in the form of a solenoid, so that the steering pressure is varied directly in response to changes in steering torque and vehicle speed.
A related invention described in my patent No. 4,561,521 includes a variable assist steering gear mechanism that uses a solenoid valve for controlling the rate of flow to an auxiliary valve section of the rotary assembly. The auxiliary valve section is in parallel disposition with respect to the main metering valve elements of the rotary valve assembly. The solenoid valve, which responds to vehicle speed, effects a change in the flow area on the discharge side of the power steering pump. Thus a variable assist is achieved in the assembly shown in patent No. 4,561,521 by varying the flow area in series with the auxiliary valve section, which in turn controls steering pressure. The result is to effect control of the steering pressure in response to steering torque and vehicle speed.